Microporous membranes and methods for producing and using such membranes

1. A microporous membrane comprising layers, wherein:

1) at least one layer comprises a first polymer comprising an ethylene/α-olefin copolymer having a Tm of 121.0 to 125.0° C. and an Mw of 5.0×10 3 to 4.0×10 5 ,

2) at least a second layer of the membrane comprises 15 wt. % to 40 wt. % of polypropylene having an Mw≧5.0×10 5 and a Tm≧150° C., 0 wt. % to 10 wt. % of polyethylene having an Mw>1.0×10 6 , and 50 wt. % to 85 wt. % of polyethylene having an Mw≦1.0×10 6 , the weight percents being based on the weight of the second layer,

3) a third layer comprises the ethylene/α-olefin copolymer,

4) the second layer is located between the first and third layers,

5) the first and third layers each individually comprise from 3 wt. % to 50 wt. %, based on the weight of the respective first layer or third layer, and

6) the membrane has a shutdown temperature ≦130.5° C., a rupture temperature ≧170.0° C. and a normalized air permeability ≦700 seconds/100 cm 3 /20 μm.

2. The microporous membrane of claim 1 , wherein the ethylene/α-olefin copolymer has a comonomer content in the range of from about 1.0 to 5.0 mol. %, and wherein the comonomer is hexene or octene.

3. The microporous membrane of claim 1 , wherein the microporous membrane has a normalized air permeability in the range of from about 50.0 to about 7.00×10 2 seconds/100 cm 3 /20 μm.

4. The microporous membrane of claim 1 , wherein the microporous membrane attains a surface impedance of 1.0×10 4 (ohm)(cm 2 ) at a temperature ≦175° C. when subjected to heating at a rate of 35° C./min.

5. The microporous membrane of claim 1 , wherein the microporous membrane has a surface impedance of 1.0×10 4 (ohm)(cm 2 ) at a temperature of 150° C. to 160° C. when subjected to heating at a rate of 35° C./min.

6. The microporous membrane of claim 1 , wherein the microporous membrane has a normalized pin puncture strength ≧3000 mN/20 μm and a porosity of 20 to 50%.

7. A multilayer microporous membrane,

first and third layers individually comprising 20.0 wt. % to 30.0 wt. % of an ethylene/α-olefin copolymer having an Mw from 1.0×10 4 to 7.0×10 4 , 45.0 wt. % to 70.0 wt. % of an polyethylene having an Mw≦1.0×10 6 , and 0.0 wt. % to 35.0 wt. % of a polyethylene having an Mw>1.0×10 6 ; based on the weight of the first and third layers, respectively, and

a second layer located between the first and third layers, comprising 20.0% to 80.0 wt. % of a polypropylene having an Mw of 1.1×10 6 and a heat of fusion >110.0 J/g, 0 wt. % to 10.0 wt. % of a polyethylene having an Mw>1.0×10 6 and 20.0 wt. % to 70.0 wt. % of a polyethylene having an Mw>1.0×10 6 , the weight percents being based on the weight of the second layer; the membrane having a shutdown temperature ≦130.5° C. and a rupture temperature ≧170.0° C.

8. A method of producing a microporous membrane comprising:

a. combining at least a first polymer and at least one first diluent, the first polymer comprising an ethylene/α-olefin copolymer having a Tm of 121.0° C. to 125.0° C. and an Mw of 5.0×10 3 to 4.0×10 5 ;

b. combining at least a second polymer and at least a second diluent, the second polymer comprising a polypropylene having an Mw≧5.0×10 5 and a Tm≧150° C., a polyethylene having an Mw>1.0×10 6 and a polyethylene having an Mw≦1.0×10 6 , wherein the amount of polypropylene is 15 wt. % to 40 wt. %, the amount of polyethylene having an Mw>1.0×10 6 is 0 wt. % to 10 wt. % and the amount of polyethylene having an Mw 50 wt. % to 85 wt. %, the weight percents being based on the total polymer weight of the composition;

c. combining at least a third polymer comprising an ethylene/α-olefin copolymer having a Tm of 121.0° C. to 125.0° C. and an Mw of 5.0×10 3 to 4.0×10 5 with at least one third diluent;

d. forming a multilayer extrudate from the combined first polymer, the polypropylene second polymer and the diluents, the extrudate having a first layer containing the first polymer, a second layer containing the second polymer, and a second layer located between the first and third layers containing the first polymer; and

e. removing at least a portion of the first, second, and third diluents from the stretched multilayer extrudate to produce the membrane.

9. The method of claim 8 , further comprising combining at least a third polymer comprising a polymer having a Tm in the range of from 115.0° C. to 130.0° C. and an Mw in the range of from 5.0×10 3 to 4.0×10 5 with at least one third diluent.

10. The method of claim 8 , wherein the first and third polymers comprise a polyolefin.

11. The method of claim 8 , wherein the first polymer comprises an ethylene/α-olefin copolymer having a Tm of ranging from 121.0 to 125.0° C., and wherein the second polypropylene comprises a polypropylene having an Mw≧5.0×10 5 and a Tm≧150° C.

12. The method claim 8 , further comprising stretching the extrudate before step (d) and removing at least a portion of any volatile species from the membrane during or after step (d).

13. The method of claim 9 , wherein:

(a) the amount of first polymer combined with first diluent is in the range of about 15 to 40 wt. % and the amount of first diluent is in the range of 85 to 60 wt. %, both weight percents being based on the combined first polymer and first diluent;

(b) the amount of third polymer combined with third diluent is in the range of about 15 to 40 wt. % and the amount of third diluent is in the range of 85 to 60 wt. %, both weight percents being based on the combined third polymer and third diluent; and

(c) the amount of the polypropylene combined with second diluent is in the range of about 15 to 40 wt. % and the amount of second diluent is in the range of 85 to 60 wt. %, both weight percents being based on the combined polypropylene and second diluent.

14. The method of claim 8 , wherein the multilayer extrudate further comprises a polyethylene having an Mw≧1.01×10 6 .

15. The method of claim 8 , wherein the multilayer extrudate further comprises a polyethylene having an Mw≦1.0×10 6 .

16. The method of claim 9 , wherein:

the first and third layers contain 3 to 50 wt. % of the polymer having a Tm in the range of from 115.0° C. to 130.0° C. and an Mw in the range of from 5.0×10 3 to 4.0×10 5, 0 to 25 wt. % of polyethylene having an Mw≧1.0×10 6 , 40 to 97 wt. % of a polyethylene having an Mw≦1.0×10 6 , based on the total weight of polyolefin in the first and third layers respectively; and

the second layer contains 15 to 40 wt. % of the polypropylene, and 0 to 10 wt. % of an independently selected polyethylene having an Mw≧1.0×10 6 , and 50 to 85 wt. % of a polyethylene having an Mw≦1.0×10 6 , based on the total weight of polyolefin in the second layer,

wherein the first, second, and third diluents are the same.

17. The method of claim 16 , wherein the polyethylene having an Mw≧1.0×10 6 in the first and third layers is the same as the polyethylene having an Mw≧1.0×10 6 in the second layer.

18. The method of claim 8 , further comprising cooling the multilayer extrudate following step (c).